(1) Field of the Invention
The present invention relates to a housing for fluid lubrication bearing apparatuses and a housing for hydrodynamic bearing apparatuses, and a method for producing the same. A fluid lubrication bearing apparatus or a hydrodynamic bearing apparatus comprising this housing is suitably used in spindle motors for information appliances, for example, magnetic disk apparatuses such as HDD, optical disk apparatuses such as CD-ROM, CD-R/RW, DVD-ROM/RAM, magneto-optic disk apparatuses such as MD and MO, polygon scanners motors of laser beam printers (LBP), and other small motors.
(2) Description of the Related Art
High rotational accuracy, enhanced speed, cost reduction, noise reduction, etc., are required for the above various kinds of motors. One of the components which determine these required performances is a bearing which supports a spindle of such motors. In recent years, the use of a fluid lubrication bearing having excellent characteristics in the above required performance is examined or such a bearing is actually used.
Fluid lubrication bearings of this type can be roughly classified into hydrodynamic bearings comprising a hydrodynamic pressure producing means which produces the hydrodynamic pressure a fluid (for example, lubricating oil or the like) in a bearing gap, and so-called cylindrical bearings (bearings having a perfectly circular bearing face) which do not have a hydrodynamic pressure producing means.
For example, in some fluid lubrication bearing apparatuses integrated into spindle motors of disk drive units such as HDD, both a radial bearing portion which supports a rotational member in the radial direction and a thrust bearing portion which supports the rotational member in the thrust direction are constituted of hydrodynamic bearings. Known thrust bearing portions in the fluid lubrication bearing apparatuses (hydrodynamic bearing apparatus) of this type include, for example, that which comprises hydrodynamic grooves formed as a hydrodynamic pressure producing part on one of both end faces of a flange portion of a shaft portion provided at the rotational member, faces opposing these faces (end faces of the bearing sleeve, the upper end face at the bottom of the housing, or a thrust member fixed on the housing, end faces of a lid member, etc.), or that which comprises a thrust bearing gap formed between both faces (for example, refer to Japanese Unexamined Patent Publication No. 2003-239951).
Moreover, there is a known bearing in which grooves (circulation grooves) through which a fluid flows in the axial direction are provided on the outer circumferential surface of the bearing sleeve for the purpose of appropriately maintaining the pressure of the fluid (maintaining the pressure balance with other portions) in a region apart from the side which is open to the air (for example, a thrust bearing portion positioned on the inside on the side opposite to the sealing space of the housing) such as a sealing space (refer to for example, Japanese Unexamined Patent Publication No. 2003-307212).
Bearing apparatuses of this type are constituted of a housing, bearing sleeve, shaft member and various other parts. To ensure high bearing performance required for improved performance of information appliances, efforts for increasing the processing accuracy and assembly precision of each part have been made. In contrast, with the trend of a decrease in the prices and the miniaturization of information appliances, a demand for cost reduction and miniaturization in this type of bearing apparatuses is increasing.
In particular, to meet the requirement for miniaturization of recent information appliances associated with their portability, size reduction of each component of bearing apparatuses is examined. For example, for size reduction of the bearing sleeve in a cylindrical shape, thinning is indispensable. However, providing circulation grooves on the outer periphery of the bearing sleeve accordingly may cause adverse effects described below.
A bearing sleeve is normally formed of a sintered metal, and a hydrodynamic pressure producing part such as hydrodynamic grooves is formed on its inner periphery by sizing after sintering. After the sizing, a bearing sleeve removed from a die or the like causes spring back, and its outer periphery is displaced (expanded) to the outer diameter side. During the sizing, since a circulation groove region of the outer circumferential surface is not in contact with the die or the like and therefore pressure is not exerted on the inner diameter side, the amount of its spring back becomes smaller than in other places. By adjusting the number of the circulation grooves (for example, three), imbalance in the amount of spring back can be decreased to a certain degree. However, as mentioned above, the thinner the wall thickness becomes, the more noticeable the imbalance in the amount of spring back becomes. In this manner, the inner circumferential surface and outer circumferential surface of the bearing sleeve after the sizing are not in the shape of a perfect circle, but its cross section is in the shape of an ellipse whose first diameter lies in the vicinity of the circulation grooves or an approximate polygon. Accordingly, variation in the circumferential direction of a radial bearing gap between the bearing sleeve and the shaft portion becomes great, which may fail to provide stable bearing rigidity.
Recently, to deal with an increase information recording density in information appliances and rotational speed, even higher rotational accuracy is demanded for spindle motors for the above information appliances. To meet this request, even higher rotational accuracy is also required for hydrodynamic bearing apparatuses integrated into the above spindle motors.
To improve rotational accuracy (bearing performance) of hydrodynamic bearing apparatuses, it is important to very accurately control the radial bearing gap and thrust bearing gap in which hydrodynamic pressure occurs. Accordingly, high dimensional accuracy is required for component parts of the hydrodynamic bearing apparatus relating to the formation of the bearing gaps mentioned above.
In contrast, recently, to achieve size reduction and reduction of the number of parts of the bearing apparatus, forming the thrust bearing gap mentioned above between an end face of the housing and an end face of the rotational member (for example, disk hub) opposing this is examined.
In this case, to achieve high rotational accuracy of the bearing apparatus, high dimensional accuracy is required not only for the shaft member and bearing sleeve, but also for an end face region which serves as a thrust bearing face of the housing. Alternatively, excellent high shape accuracy is also required among other faces constituting the housing. However, currently existing processing methods are unable to increase the accuracy since processing costs are significantly increased.